Different types of surge arresters are today used in switchgears in order to protect power network equipment against incoming overvoltages. A surge arrester is connected between a live wire and ground and may comprise one or more gapless surge arrester modules with varistor blocks of metal oxide, for instance zinc oxide, arranged between two end electrodes. In a varistor block of metal oxide, the electrical resistance is high at low voltages but low at high voltages. When the voltage level in the live wire exceeds a critical value, the surge arrester will allow the electric current to be conducted to ground through the varistor blocks, whereby the overvoltage is reduced.
When the normal operating voltage in the live wire is so high that a single varistor block is not capable of resisting the operating voltage, several varistor blocks are connected in series in a stack between the end electrodes of the surge arrester module. To carry large currents through a stack of varistor blocks and to give the surge arrester module a good stability, a sufficient contact pressure must be maintained between the varistor blocks. The required contact pressure between the varistor blocks may be achieved by means of elongated clamping members of electrically insulating material which are connected to the end electrodes and prestressed so as to press the end electrodes towards each other in the axial direction of the surge arrester module and thereby achieve contact pressure between the varistor blocks. The clamping members may for instance have the form of endless loops, as shown in U.S. Pat. No. 5,517,382 A, U.S. Pat. No. 5,912,611 A and WO2012098250 A1, or rod-like elements, as shown in U.S. Pat. No. 5,291,366 A and U.S. Pat. No. 6,777,614 A.
A surge arrester to be used in a power network of high system voltage is often formed by one or more groups of surge arrester modules of the above-mentioned type, wherein each group comprises two or more surge arrester modules mounted in parallel with each other between a lower support member and an upper support member. The support members may for instance have the form of plates or beams. In order to adapt the surge arrester to the system voltage, two or more such groups of surge arrester modules may be stacked on top of each other and fixed to each other with the surge arrester modules in one group connected in series with the surge arrester modules of each adjacent group.
If a multi-module surge arrester of the above-mentioned type is placed on a foundation and consequently supported from below, some of the surge arrester modules may be subjected to an axial tensile force when other surge arrester modules, due to uneven load distribution on the surge arrester, are axially compressed. Such an uneven load distribution on the surge arrester may for instance occur due to uneven ice formation on the surge arrester, heavy wind or earthquakes. Heavy connecting cables hanging obliquely from the top of the surge arrester may also cause an uneven load distribution on the surge arrester, particularly when the cables are trembling due to overvoltages. In the worst case, the axial tensile force on a surge arrester module may become so high that the prestress force of the clamping members is lost, which in its turn would result in an unacceptable loss of contact pressure between the varistor blocks in the surge arrester module.
In order to avoid high tensile forces in the surge arrester modules of a multi-module surge arrester of the above-mentioned type and thereby prevent a loss of contact pressure between varistor blocks of the surge arrester modules, the surge arrester is normally suspended through the uppermost group of surge arrester modules, for instance in a bus bar. However, it is not always possible or desirable to use a suspended surge arrester and there is therefore a need for an alternative solution to the above-mentioned problem, to thereby make it possible to place a multi-module surge arrester of the above-mentioned type on a foundation without running the risk of losing contact pressure between varistor blocks of the surge arrester modules due to uneven load distribution on the surge arrester. Besides, high tensile forces may also occur in a suspended surge arrester and cause loss of contact pressure between varistor blocks of a suspended surge arrester if the suspended surge arrester is big and heavy and/or supports additional equipment or long cables.